LSN is not trivial.
Here is some unverified calculations I did on the problem of scaling nat.
One of my colleagues here (Shane Alcock) did some research into "Service Provider NAT" based off passive traces from a New Zealand Residential ISP[1]. By passively looking at connections he investigated how you could dimension a NAT box for an ISP. His research is available here http://www.wand.net.nz/~salcock/spnat/tech_report.pdf . If walls of text scare you (why are you reading this mailing list then?) skip through and look at the graphs (page 3 onwards) [1]: Contrary to Joe Abley's belief, I'm not aware of any major DSL provider in NZ that is doing NAT inside their network -- although almost all of the CPE's in New Zealand do NAT you, so as an end user you usually do end up behind NAT, but it is one under your own control, and can be eliminated with a careful choice of CPE. There are some wifi providers, and some 3G APN's that will provide you with ISP NAT.